How the Date of Easter Is Calculated

Easter is a moveable feast because it is tied to the Jewish Passover, which follows the lunisolar Hebrew calendar. Early Christians disagreed over how to fix it, and the Council of Nicaea in 325 is traditionally credited with settling that Easter should be celebrated on a Sunday, independently of the Jewish calculation, with the Alexandrian method gradually becoming standard.

The rule is that Easter falls on the first Sunday after the paschal full moon, the ecclesiastical full moon on or after 21 March. The full moon here is not the astronomical one but a value from tables built on the 19-year Metonic cycle. A year's position in that cycle is its golden number, the remainder of the year divided by 19, plus one.

In the Gregorian system Western Easter falls between 22 March and 25 April. Orthodox churches use the same principle on the Julian calendar, so their Easter falls between 4 April and 8 May in Gregorian terms, and the two Easters coincide in some years and are up to five weeks apart in others.

The calculation tables, or computus, were refined by Dionysius Exiguus, whose Easter table of 525 also introduced Anno Domini dating, and later by the Gregorian reformers, who added solar and lunar corrections for century years. The complete Gregorian sequence of Easter dates repeats only after 5,700,000 years.